Re: [PATCH 3/7] block: copy offload support infrastructure

[Kanchan Joshi <joshiiitr@gmail.com>]

On Thu, Aug 19, 2021 at 12:05 AM Martin K. Petersen
<martin.petersen@oracle.com> wrote:
>
>
> > Native copy offload is not supported for stacked devices.
>
> One of the main reasons that the historic attempts at supporting copy
> offload did not get merged was that the ubiquitous deployment scenario,
> stacked block devices, was not handled well.
>
> Pitfalls surrounding stacking has been brought up several times in
> response to your series. It is critically important that both kernel
> plumbing and user-facing interfaces are defined in a way that works for
> the most common use cases. This includes copying between block devices
> and handling block device stacking. Stacking being one of the most
> fundamental operating principles of the Linux block layer!
>
> Proposing a brand new interface that out of the gate is incompatible
> with both stacking and the copy offload capability widely implemented in
> shipping hardware makes little sense. While NVMe currently only supports
> copy operations inside a single namespace, it is surely only a matter of
> time before that restriction is lifted.
>
> Changing existing interfaces is painful, especially when these are
> exposed to userland. We obviously can't predict every field or feature
> that may be needed in the future. But we should at the very least build
> the infrastructure around what already exists. And that's where the
> proposed design falls short...
>
Certainly, on user-space interface. We've got few cracks to be filled
there, missing the future viability.
But on stacking, can that be additive. Could you please take a look at
the other response (comment from Bart) for the trade-offs.


-- 
Joshi